The present invention relates to solid propellant grains or the like and more particularly to a process for preparing propellant grains comprising thermoplastic binders and suspended solid particulates.
Conventional solid composite propellants have binders which utilize cross-linked elastomers in which prepolymers are cross-linked by chemical curing agents. As outlined in U.S. Pat. No. 4,361,526, there are important disadvantages to cross-linked elastomers. Cross-linked elastomers must be cast within a short time after addition of the curing agent, which time period is known as the "pot life". Disposal of a cast cross-linked propellant composition is difficult, except by burning, which poses environmental problems.
As an alternative to cross-linked elastomer binders, U.S. Pat. No. 4,361,526 proposes to use a thermoplastic elastomeric binder which is a block copolymer of a diene and styrene, the styrene blocks providing a meltable crystalline structure and the diene blocks imparting rubbery or elastomeric properties to the copolymer. In order to prepare a propellant composition using the copolymer, the copolymer is dissolved in an organic solvent, such as toluene, and the solids and other propellant ingredients are added. The solvent is then evaporated, leaving a rubbery solid which may be divided into pellets suitable for casting or other processing.
A disadvantage of formulating a propellant using a thermoplastic elastomeric binder which must be dissolved in a solvent is that the propellant grain cannot be cast in a conventional manner, e.g., into a rocket motor casing. Further, solvent-based processing presents problems with respect to removal and recovery of the solvent. Organic solvents, such as toluene., present certain hazards to the immediate work area and to the larger environment, necessitating various precautions to be taken with regard to processing such propellant formulations.
It has also been proposed to produce thermoplastic elastomeric propellants in which the solid particulates and thermoplastic elastomer are fused in a high-shear mixer and the fused mixture poured or extruded into a casing or mold. There are important disadvantages with such techniques. Because of the high-solids loading of propellant formulations, viscosities tend to be quite high, making mixing difficult, and, in many cases, impossible on a large scale. Localized overheating in high-shear apparatus can cause ignition and catastrophic combustion of these high-energy formulations.
As an alternative to high-shear mixing and/or extrusion, U.S. Pat. No. 4,764,316 proposes to dry blend thermoplastic elastomer particles and energetic particles, pack the blended particles into a mold or casing, then fuse the particulates and thermoplastic elastomer together. A disadvantage of this method is the possibility of voids in the fused propellant.
There exists a need for improved processes for producing propellant grains with thermoplastic elastomer binders.
Accordingly, it is an object of the present invention to provide a castable, thermoplastic composite rocket propellant.
It is another object of this invention to provide a process for producing a thermoplastic composite rocket propellant.
Other objects and advantages of the present invention will become apparent to those persons skilled in the art from a reading of the following description of the invention.